Military event revealed after over a year

09 May 2007

The US Nuclear Regulatory Commission's (NRC's) annual report on 'abnormalevents' has revealed a serious spill of high-enriched uranium (HEU)solution at a facility used to make nuclear fuel for naval reactors. No workers were hurt.

A recently published NRC report to Vice President Dick Cheney, published in the US Federal Register, details the events from FY2006 which commissioners consider to be significant from the standpoint of public health and safety. One passage details the HEU spillage, which took place at Nuclear Fuel Services' (NFS's) fuel fabrication facility in Erwin, Tennessee, on 6 March 2006.

HEU is uranium enriched to the point that between 20-90% of the material is U-235, the isotope usually split to release energy. Military reactors for naval power and propulsion are fuelled by HEU fuel, which is made at certain designated facilities under NRC oversight.

The event occurred when 35 litres of HEU solution leaked into a glovebox while being transferred. The glovebox would normally have been sealed tightly, but workers had recently moved it and failed to reseal it correctly. The HEU solution was thus able to leak from the glovebox onto the floor. The NRC noted that a nearby elevator pit represented an 'uncontrolled accumulation point'.

The NRC report states that criticality would have been possible both in the glovebox and in the elevator pit, but not that criticality ever occurred. It continues to say that "the total volume of the transfer would have been more than enough for criticality in the glovebox or the elevator pit."

Criticality is when a chain reaction releasing heat and large amounts of radiation is initiated within a body of material containing a critical mass of U-235 or another fissile element such as U-233, Pu-239, or Pu-241. This is normally made impossible by very strictly limiting amounts and locations of materials and also by the geometry of vessels made to contain them.

The report says that if a criticality accident had occurred in the glovebox or the pit "it is likely that at least one worker would have received an exposure high enough to cause acute health effects or death."

In response to the event, NFS stopped HEU processing in the area of the event and removed the processing enclosure and all its pipework. NFS also filled in the elevator pit with concrete and conducted an extensive review to identify any similar configuration issues.

Further information

US Nuclear Regulatory Commission

US Federal Register: Report to Congress on Abnormal Occurences Fiscal Year 2006

---